A motor vehicle usually comprises contact variables, which must be monitored periodically in order to determine the state of the vehicle.
Examples of a contact variable include: a door closing contact, a luggage compartment closing contact, a headlight activation lever contact, a windscreen wiper activation lever contact, etc. A contact variable has two states: an open state and a closed state. To this end, a contact variable is traditionally constituted by a two-state switch, as will be detailed hereinafter.
Usually, the vehicle comprises an electronic read device adapted to monitor the contact variables and to respond in accordance with the states thereof. By way of example, if the luggage compartment of the vehicle is open, the read device can order the activation of a warning light on the dashboard of the vehicle so as to warn the driver.
The read device monitors the state of the contact variables periodically so as to limit its electrical consumption. Read intervals that are alternated with sleep intervals are also distinguished, that is to say a period comprises a read interval and a sleep interval.
The read device, also known to a person skilled in the art by the name ECU or “electronic control unit”, usually comprises a read module provided for example in the form of a microcontroller.
The read module comprises a plurality of read ports connected to the contact variables to be monitored. In practice, the contact variables are connected to the read ports by an electric circuit.
In a known embodiment called “low-side input”, the electric circuit comprises a supply point connected to a general power source via a switch adapted to be controlled by the read module in such a way that the switch is closed during a read interval and open during a sleep interval. In other words, the switch makes it possible to supply the electric circuit periodically so as to limit the electrical consumption of the read device.
By way of example, FIG. 1 shows a “low-side input” device, in which a read module MOD is connected to a power source Vcc in order to electrically power said module, for example with an electric voltage of approximately 5 volts. In this example, the read module MOD comprises a read port P1 connected to a contact variable Var-1 by an electric circuit CE comprising a supply point A. This supply point is connected to a general power source SGEN via a read switch INT, controlled by the read module MOD, making it possible to electrically connect or disconnect the electric circuit CE to/from the general power source SGEN.
By way of example, with reference to FIG. 1, the electric circuit CE comprises:                an injection branch BI, connecting the read port P1 to the contact variable Var-1, comprising an injection resistor Rs1 and        a supply branch BA, connecting the contact variable Var-1 to the supply point A, comprising a power supply resistor Rp1.        
The read module MOD periodically controls the switch INT so as to close said switch, thus connecting the electric circuit CE to the general power source SGEN, as illustrated in FIG. 2. Following the closure of the switch INT, an electric current Iinj-1 is generated in the injection branch BI of the electric circuit CE when the contact variable Var-1 is open. The electric current generated Iinj-1 is received by the read port P1 of the read module MOD. When the contact variable Var-1 is closed, no electric current circulates in the injection branch, the current circulating instead in the supply branch. It is also sufficient to measure the electric voltage of the read port P1 to determine whether the monitored contact variable Var-1 is open or closed.
FIG. 3 shows the monitoring of a plurality of contact variables Var-1, . . . Var-i, . . . Var-n connected to a plurality of read ports P1, . . . Pi, . . . Pn. The electric circuit CE makes it possible to connect each contact variable to its read port by an injection branch BI-1, BI-i, BI-n, whereas each contact variable Var-1, Var-i, Var-n is connected to the supply point A of the electric circuit CE by a supply branch BA-1, BA-i, BA-n. As illustrated in FIG. 3, the injection branches BI-1, BI-i, BI-n and the supply branches BA-1, BA-i, BA-n comprise, respectively, an injection resistor Rs1, Rsi, Rsn and a power supply resistor Rp1, Rpi, Rpn.
Similarly to the first example, during a read interval, when the switch INT is closed, an electric current Iinj-1, Iinj-i, Iinj-n is generated in each injection branch BI-1, BI-i, BI-n if the contact variable Var-1, Var-i, Var-n is open. Each electric current generated Iinj-1, Iinj-i, Iinj-n is received by a read port P1, Pi, Pn. The read module MOD can thus determine the state of the contact variables Var-1, Var-i, Var-n.
During a sleep interval, when the switch INT is open, no electric current Iinj is generated in the injection branches BA-1, BA-i, BA-n.
In the case of a wiring fault or malfunction, a short circuit may appear and may connect a contact variable to a battery voltage ABAT of the vehicle as illustrated in FIG. 4. In practice, the battery voltage ABAT is approximately 12 volts.
If the malfunction appears during a sleep interval, the battery voltage ABAT basically performs the same function as the power source SGEN and an electric current Iinj-1, Iinj-i, Iinj-n is thus generated in each injection branch BI-1, BI-i, BI-n if the contact variable Var-1, Var-i, Var-n is open for said injection branch BI-1, BI-i, BI-n. In other words due to the malfunction, the read ports P1, Pi, Pn of the read module MOD receive the electric currents of all the injection branches BI-1, BI-i, BI-n while the read module MOD is in sleep mode.
In practice, the read module MOD is not affected by the sum of received currents ΣInj as long as this sum is less than the value of the current consumed IC by the read module MOD. During a read interval, the electric current Iinj-1, Iinj-i, Iinj-n generated in each injection branch BI-1, BI-i, BI-n via the short circuit is thus less than the current consumed IC by the read module MOD. In other words, the read module MOD consumes the current generated by the short circuit.
A disadvantage appears for last-generation vehicles, of which the read module MOD comprises a mode of use implemented when the vehicle is started, and also a sleep mode implemented when the vehicle is switched off. In sleep, the current consumed IC by the read module MOD is very low. As a result, the sum of received currents ΣInj may exceed the value of the consumed current IC, which affects the power supply of the read module MOD and may lead to damage of the electronic components of the read module MOD.
In order to eliminate this disadvantage, an immediate solution would be to block the electric current received by the read ports of the read module MOD by means of diodes, however the use of additional components complicates the device and leads to additional manufacturing costs.